In recent years, there have been proposed internal combustion engines which can be operated by the use of a plurality of kinds of fuels. In such internal combustion engines, there has been proposed a technique in which a gas fuel (CNG: Compressed Natural Gas) is used during a period of time from engine starting until a catalyst for exhaust gas purification becomes activated, and a liquid fuel (gasoline) is used after the exhaust gas purification catalyst has been activated (see, for example, a first patent document).
In addition, there has also been proposed a technique in which in an internal combustion engine capable of using a plurality of kinds of fuels, a liquid fuel is used at the time of carrying out NOX reduction treatment of an NOX storage reduction catalyst which is arranged in an exhaust system of the internal combustion engine (see, for example, a second patent document).